1. Field of the Invention
This invention relates to an optical position detector for determining a position of a point of light source directly or indirectly emitting a light by the use of an image pickup device. More particularly, it relates to a technique for improving the accuracy of such an optical position detector as mentioned above.
2. Description of the Related Art
Heretofore, in the surveillance of the behavior of a land slide for example, targets composed of a point light source are regularly placed on a slope plane of a mountain. The beams of light directly or indirectly emitted from the points of light source are captured by a TV camera installed at a remote site. Then, image signals are analyzed, thereby detecting the positions of the individual targets. Particularly in cases where the surveillance requires data on three-dimensional displacement of the individual targets, the stereo method which uses two TV cameras is frequently adopted.
For the system of surveillance of this nature, in order to acquire an improvement in the accuracy of position determination, the TV camera used therein is required to possess improved resolution. For the purpose of improving the resolution, the number of pixel elements of the image pickup device such as CCD which is incorporated in the TV camera must be greatly increased. This increase, however, is difficult to achieve from the standpoint of the technology of manufacture.
A technique for apparently improving the resolution without requiring an increase in the number of pixel elements of the image pickup device has been proposed by JP-A-60-218,002, for example. This technique comprises attaching a cross filter to a lens of a TV camera, converting a light beam from a target into streaks of a cruciform image by means of the cross filter, and projecting the cruciform image on a light receiving surface of the image pickup device. The position of a point light source which forms the target is determined by subjecting the projected image to image processing and consequently finding by computation a cross point of the cruciform image. This conventional method, however, fails to produce clear streaks of the cruciform image unless the light beam from the point light source has a certain degree of intensity. Thus, it is at a disadvantage in being deficient in sensitivity.